In general, an SOFC comprises a pair of electrodes (anode and cathode) that are separated by a ceramic, solid-phase electrolyte. To achieve adequate ionic conductivity in such a ceramic electrolyte, the SOFC operates at an elevated temperature, typically in the order of between about 700° C. and 1000° C. The material in typical SOFC electrolytes is a fully dense (i.e. non-porous) yttria-stabilized zirconia (YSZ) which is an excellent conductor of negatively charged oxygen (oxide) ions at high temperatures. Typical SOFC anodes are made from a porous nickel/zirconia cermet while typical cathodes are made from magnesium doped lanthanum manganate (LaMnO3), or a strontium doped lanthanum manganate (also known as lanthanum strontium manganate (LSM)). In operation, hydrogen or carbon monoxide (CO) in a fuel stream passing over the anode reacts with oxide ions conducted through the electrolyte to produce water and/or CO2 and electrons. The electrons pass from the anode to outside the fuel cell via an external circuit, through a load on the circuit, and back to the cathode where oxygen from an air stream receives the electrons and is converted into oxide ions which are injected into the electrolyte. The SOFC reactions that occur include:
Anode reaction:H2+O=→H2O+2e−CO+O=→CO2+2e−CH4+4O=→2H2O+CO2+8e−
Cathode reaction:O2+4e−→2O=
Known SOFC designs include planar and tubular fuel cells. Applicant's own PCT application no. PCT/CA01/00634 discloses a method of producing a tubular solid oxide fuel cell by electrophoretic deposition (EPD). The fuel cell comprises multiple concentric layers, namely an inner electrode layer, a middle electrolyte layer, and an outer electrode layer. The inner and outer electrodes may suitably be the anode and cathode respectively, and in such case, fuel may be supplied to the anode by passing through the tube, and air may be supplied to the cathode by passing over the outer surface of the tube. Multiple such fuel cells can be electrically grouped together into stacks to increase power production density.
Because SOFCs can only operate at elevated temperatures, they must be heated before they can generate electricity. During operation, the fuel cells produce electricity and heat. The generated heat can in some instances be used to maintain the fuel cells at their operating temperature; however, in very small scale applications or in other instances, the fuel cells cannot generate enough heat on their own, or there is not enough thermal insulation around the fuel cells to maintain the fuel cells at their operating temperature. In such instances, heat must be provided from an external source. External heating must also be provided at start up, when the fuel cells are not generating any heat.
It is therefore desirable to provide a fuel cell system that can supply sufficient heat to the fuel cells in the system during start up and during operation. In particular, it is desirable to provide a system that can provide such heat in a relatively quick and efficient manner.